CMOS image sensor

ABSTRACT

A CMOS (Complementary Metal-Oxide Semiconductor) image sensor is provided. A CMOS image sensor includes a first light-receiving unit converting light into charge, a first floating diffusion region, in which a first potential corresponding to the converted amount of charge is generated and a second floating diffusion region, to which the charge in the first floating diffusion region is transmitted, and in which a second potential is generated, wherein a wide dynamic range signal is acquired from the first floating diffusion region, a high-sensitively signal is acquired from the second floating diffusion region, and the acquired signals are synthesized and output.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Korean Patent Application No.10-2007-0077016 filed on Jul. 31, 2007 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor technology, and, inparticular, to a CMOS (Complementary Metal-Oxide Semiconductor) imagesensor that has an expanded dynamic range.

2. Description of the Related Art

In recent years, high-resolution camera-equipped apparatuses, such asdigital cameras, camera-equipped cellular phones, and surveillancecameras, have become widespread. As an imaging device for such a camera,a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-OxideSemiconductor) image sensor is used.

The CMOS image sensor has features of ease of manufacturing and low costcompared with the CCD, and thus it is popular in solid-state imaging.Further, a unit pixel of the CMOS image sensor is composed of MOStransistors, and thus it can be implemented in a smaller area than thatof the CCD, thereby providing high resolution. In addition,signal-processing logic can be formed in an image circuit, in whichpixels are formed, such that the image circuit and the signal-processingcircuit can be incorporated into a single body.

Since the CMOS image sensor generally has a dynamic range ofapproximately 60 dB, there is a limit to generating an image in a wideilluminance range. For this reason, in a screen having a bright imageand a dark image, a bright portion may be saturated and become white,and a dark portion may not be expressed.

In addition, as a digital camera or a camera-equipped cellular phone isreduced in size, low-voltage driving is performed due to demands forreducing a unit area in the pixels of the image sensor and realizing lowpower consumption, which makes it difficult to ensure a sufficientdynamic range.

In the related art, in order to solve the above-described problems, thestructure shown in FIG. 1 is used to expand the dynamic range of theimage sensor.

FIG. 1 is a circuit diagram showing a unit pixel having a general 4-Tstructure in a CMOS image sensor.

Referring to FIG. 1, the pixel having a 4-T structure is composed of onephotodiode (PD) 110, and four NMOS transistors, that is, a transfertransistor (Tx) 120, a reset transistor (Rx) 122, a drive transistor(Dx) 124, and a select transistor (Sx) 126.

In a state where the transfer transistor (Tx) 120 is turned off, iflight is irradiated onto the surface of the photodiode (PD) 110, holesand electrons are separated. Then, the holes flow to a ground to be thenremoved, and electrons accumulate in the photodiode (PD) 110.

The transfer transistor (Tx) 120 functions as a transmission channel toapply a predetermined voltage to a gate 121 of the transfer transistor(Tx) 120, and to transfer the electrons accumulated in the photodiode(PD) 110 by light to a floating diffusion region (FD) 130. Further, thetransfer transistor (Tx) 120 performs a reset function to completelyremove the electrons from the photodiode (PD) 110.

The reset transistor (Rx) 122 resets the floating diffusion region (FD)130 by setting the potential of the floating diffusion region (FD) 130to a desired value and eliminating charge. That is, the reset transistor(Rx) 122 eliminates the charge that has accumulated in the floatingdiffusion region (FD) 130 for signal detection.

The drive transistor (Dx) 124 operates according to the chargeaccumulated in the floating diffusion region (FD) 130, and functions asa buffer amplifier having the configuration of a source follower. Theselect transistor (Sx) 126 is switched for addressing.

If charge accumulates in the photodiode (PD) 110, a high voltage isapplied to a gate of the reset transistor (Rx) 122 to set the voltage ofthe floating diffusion region (FD) 130 to V_(DD), and then acorresponding voltage value is read. Next, a high voltage is applied tothe gate of the transfer transistor (Tx) 120 to transfer the charge thathas accumulated in the photodiode (PD) 110 to the floating diffusionregion (FD) 130, a corresponding voltage value is read, and subsequentlya difference between the read voltage values is read.

In this structure, in order to expand the dynamic range, the capacitanceof the floating diffusion region (FD) 130 is increased to receive thecharge from the photodiode (PD) 110 without overflow.

However, if the capacitance is increased, sensitivity of the CMOS imagesensor is decreased, and a dark image may not be expressed. Therefore,it is not desirable to simply increase the capacitance of the floatingdiffusion region (FD) 130.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a CMOS image sensor, inwhich a plurality of floating diffusion regions are provided in a pixel,having the advantage of obtaining an expanded dynamic range withoutsacrificing sensitivity.

Objects of the present invention are not limited to those mentionedabove, and other objects of the present invention will be apparent tothose skilled in the art through the following description.

According to the embodiments of the present invention, a plurality offloating diffusion regions are provided in a pixel to have differentcapacitance, and thus an expanded dynamic range can be obtained withoutsacrificing sensitivity.

According to the embodiments of the present invention, the floatingdiffusion regions are separated from each other. Therefore, at lowilluminance, a vivid image can be obtained with high sensitivity. Inaddition, at high illuminance, a vivid image can be obtained withoutcausing an image to be saturated and whitened.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a circuit diagram showing a unit pixel having a general 4-Tstructure in a CMOS image sensor;

FIG. 2 is a circuit diagram showing a unit pixel of a CMOS image sensorhaving two floating diffusion regions according to an embodiment of thepresent invention;

FIG. 3 is a timing chart illustrating the operation of the circuit shownin FIG. 2;

FIG. 4 is a circuit diagram showing the structure of a CMOS image sensoraccording to another embodiment of the present invention;

FIG. 5 is a circuit diagram showing a unit pixel of a CMOS image sensoraccording to still another embodiment of the present invention; and

FIG. 6 is a timing chart illustrating the operation of the circuit shownin FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Advantages and features of the present invention and methods ofaccomplishing the same may be understood more readily by reference tothe following detailed description of preferred embodiments and theaccompanying drawings. The present invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete and will fullyconvey the concept of the present invention to those skilled in the art,and the present invention will only be defined by the appended claims.

FIG. 2 is a circuit diagram showing a unit pixel of a CMOS image sensorhaving two floating diffusion regions according to an embodiment of thepresent invention.

Referring to FIG. 2, a CMOS image sensor according to an embodiment ofthe present invention has a first floating diffusion region (FD1) 230 aand a second floating diffusion region (FD2) 230 b per unit pixel. FD1230 a and FD2 230 b are separated from each other by a second transfertransistor (Tx2) 220 b. In addition, a first transfer transistor (Tx1)220 a is disposed between a photodiode (PD) 210 and FD1 230 a.

The photodiode (PD) 210 functions as a light-receiving unit thatconverts light into charge. It should be understood that any unit can beapplied to the present invention insofar as it is a light-receiving unitthat can convert light into charge.

FD1 230 a is connected to a gate of a first drive transistor (Dx1) 224 aand a first reset transistor (Rx1) 222 a, and FD2 230 b is connected toa gate of a second drive transistor (Dx2) 224 b and a second resettransistor (Rx2) 222 b.

A final image for a pixel is obtained by synthesizing signals Vout1 andVout2 that are output from a first select transistor (Sx1) 226 a and asecond select transistor (Sx2) 226 b.

The transfer transistors 220 a and 220 b, the reset transistors 222 aand 222 b, the drive transistors 224 a and 224 b, and the selecttransistors 226 a and 226 b shown in FIG. 2 have the same functions asthe transistors shown in FIG. 1.

Referring to FIG. 2, FD1 230 a is disposed close to the four transistorsTx1 220 a, Tx2 220 b, Rx1 222 a, and Dx1 224 a, and thus it has acapacitance larger than FD2 230 b that is disposed close to the threetransistors Tx2 220 b, Rx2 222 b, and Dx2 224 b.

At this time, the capacitance of FD1 230 a is maximized within apredetermined range to receive large amounts of charge while thesensitivity is low. Further, the capacitance of FD2 230 b is minimizedwithin the predetermined range to increase the sensitivity while notreceiving large amounts of charge.

In such a manner, a signal having a wide dynamic range with respect toilluminance but low sensitivity can be acquired in FD1 230 a, and asignal having a small dynamic range with respect to illuminance but highsensitivity can be acquired in FD2 230 b.

That is, the charge accumulated in the photodiode 210 is transmitted toFD1 230 a through the first transfer transistor (Tx1) 220 a to obtain awide dynamic range signal, and then the wide dynamic range signal isoutput as Vout1 through Dx1 224 a and Sx1 226 a. Next, the wide dynamicrange signal obtained in FD1 230 a is transmitted to FD2 230 b throughthe second transfer transistor (Tx2) 220 b to obtain a high-sensitivesignal, and then the high-sensitivity signal is outputs as Vout2 throughDx2 224 b and Sx2 226 b.

The signals Vout1 and Vout2 are synthesized, thereby obtaining the finalimage for a pixel.

FIG. 3 is a timing chart illustrating the operation of the circuit shownin FIG. 2.

Referring to FIG. 3, Sx1 226 a is turned on at time t0 when a selectioncontrol signal rises, and a column including a corresponding CMOS pixelelement is selected.

Next, Rx1 222 a is turned on at time t1 to reset FD1 230 a to V_(DD),and then a corresponding voltage value is read.

At time t2, a high voltage is applied to a gate of the Tx1 220 a totransmit the charge accumulated in the photodiode 210 to FD1 230 a, anda corresponding voltage value is read. A difference between the twovoltage values is output as a final signal value. That is, the outputsignal covers a wide range of illuminance, and thus a vivid image can beobtained with high illuminance without causing saturation.

After time t2, Sx2 226 b is turned on, and a column including acorresponding CMOS pixel element is selected. In this case, the samecolumn is selected by Sx1 226 a and Sx2 226 b.

At time t3, the Rx2 222 b is turned on to rest FD2 230 b to V_(DD), andthen a corresponding voltage value is read. Next, at time t4, a highvoltage is applied to a gate of the Tx2 220 b to transmit the chargeaccumulated in FD1 230 a to FD2 230 b, and then a corresponding voltagevalue is read. A difference between the two voltage values is output asa final signal value. In FD2 230 b, a high-sensitivity signal is outputdue to low capacitance, such that a vivid image can be obtained with lowilluminance.

As a result, the two final signal values are synthesized after a timet4, such that an illuminance range can be expanded while the sensitivityof the CMOS image sensor can be maintained.

FIG. 4 is a circuit diagram showing the structure of a CMOS image sensoraccording to another embodiment of the present invention. Referring toFIG. 4, images from two pixels of the CMOS image sensor are processed bya single circuit.

That is, an image-processing circuit block 450 shown in FIG. 4 has thesame configuration and function as the circuit shown in FIG. 2. In FIG.4, however, a first floating region (FD1) 430 a is connected to a thirdtransfer transistor (Tx3) 420 c, and Tx3 420 c is connected to a secondphotodiode (PD2) 410 b.

A first photodiode (PD1) 410 a and a second photodiode (PD2) 410 brespectively function as light-receiving units of first and secondpixels in the CMOS image sensor.

For example, charge collected by the PD1 410 a is transmitted to FD1 430a and FD2 430 b under the control of Tx1 420 a, thereby obtaining outputsignals Vout1 and Vout2 for the first pixel. In this case, since Tx3 420c does not operate, charge collected in the PD2 410 b is not transmittedto FD1 430 a.

Subsequently, the Tx1 420 a does not operate and the Tx3 420 c operates.Then, the charge collected in the PD2 410 b is transmitted to FD1 430 aand FD2 430 b, thereby obtaining output signals Vout1 and Vout2 for thesecond pixel. In this case, since the Tx1 420 a does not operate, thecharge collected in the PD1 410 a is not transmitted to FD1 430 a.

That is, a single image-processing circuit block 450 is shared by twolight-receiving units, and thus the integration of the CMOS image sensorcan be increased.

FIG. 5 is a circuit diagram showing a unit pixel of a CMOS image sensoraccording to still another embodiment of the present invention.

Referring to FIG. 5, it can be seen that the circuit shown in FIG. 5 hasthe same configuration as the circuit shown in FIG. 2, excluding acapacitor 550.

The capacitor 550 is connected to a gate of a Dx2 524 b, that is, a FD2530 b, to increase capacitance of FD2 530 b. Accordingly, FD1 530 afunctions as a high-sensitivity output unit, and FD2 530 b functions asa wide dynamic range/low-sensitivity output unit, unlike the circuitshown in FIG. 2, in which FD1 230 a functions as a wide dynamic rangeoutput signal and FD2 230 b functions as a high-sensitivity output unit.

Therefore, referring to FIG. 5, the wide dynamic range signal is outputas Vout2, and the high-sensitivity signal is output as Vout1.

FIG. 6 is a timing chart illustrating the operation of the circuit shownin FIG. 5.

Referring to FIG. 6, Sx1 526 a and the Sx2 526 b are simultaneouslyturned on at time t0 when the selection control signal rises, and acolumn including a corresponding CMOS pixel element is selected.

Next, at time t1, the reset transistor (Rx1) 522 a and the resettransistor (Rx2) 522 b are simultaneously turned on to set FD1 530 a andFD2 530 b to V_(DD), and then a corresponding voltage value is read.

At time t2, a voltage V_(h) is applied to the first transfer transistor(Tx1) 520 a, and a voltage V_(m) is applied to the second transfertransistor (Tx2) 520 b. At this time, the voltage V_(m) is lower thanthe voltage V_(h).

Subsequently, at time t2, FD2 530 b receives the excessive charge in FD1530 a, such that a high-sensitivity signal is obtained from FD1 530 a,and a low-sensitivity/wide dynamic range signal is obtained from FD2 530b. Next, the two signals are synthesized, thereby acquiring a widedynamic range/high-sensitivity signal.

Similar to the CMOS image sensor shown in FIG. 4, the circuit shown inFIG. 5 can be shared by at least two light-receiving units. This changecan be easily made by those skilled in the art from FIG. 4.

Although the present invention has been described in connection with theexemplary embodiments of the present invention, it will be apparent tothose skilled in the art that various modifications and changes may bemade thereto without departing from the scope and spirit of the presentinvention. Therefore, it should be understood that the above embodimentsare not limitative, but illustrative in all aspects.

1. A complementary metal-oxide-semiconductor (CMOS) image sensorcomprising: a first light-receiving unit to convert light into charge; afirst floating diffusion region to receive a charge from the firstlight-receiving unit which is transmitted through a first transfertransistor, to generate a first potential corresponding to the charge; asecond floating diffusion region to receive a charge from the firstfloating diffusion region which is transmitted through a second transfertransistor, to generate a second potential, wherein a wide dynamic rangesignal is acquired from the first floating diffusion region, ahigh-sensitivity signal is acquired from the second floating diffusionregion, and the acquired signals are synthesized and output, and whereinthe first floating diffusion region is connected to a first resettransistor and the second floating diffusion region is disposed betweenthe second transfer transistor and a second reset transistor, and thefirst and second floating diffusion regions are not connected to acapacitor.
 2. The CMOS image sensor of claim 1, wherein: the firstfloating diffusion region is connected to a second light-receiving unit,and the first light-receiving unit and the second light-receiving unitshare the first floating diffusion region and the second floatingdiffusion region.
 3. The CMOS image sensor of claim 1, wherein: thefirst floating diffusion region has a capacitance larger than the secondfloating diffusing region.
 4. The CMOS image sensor of claim 1, wherein:the wide dynamic range signal acquired from the first floating diffusionregion is output through a first drive transistor and a first selecttransistor, the high-sensitively signal acquired from the secondfloating diffusion region is output through a second drive transistorand a second select transistor, and the acquired signals are thensynthesized and output.
 5. The CMOS image sensor of claim 2, wherein:the first transfer transistor is disposed between the firstlight-receiving unit and the first floating diffusion region, and thesecond transfer transistor is disposed between the secondlight-receiving unit and the first floating diffusion region, and thesharing of the first floating diffusion region and the second floatingdiffusion region by the first light-receiving unit and the secondlight-receiving unit comprises: when the first transfer transistoroperates, the charge converted by the first light-receiving unit istransmitted to the first floating diffusion region and the secondfloating diffusion region, and the second transfer transistor does notoperate; and when the second transfer transistor operates, a chargeconverted by the second light-receiving unit is transmitted to the firstfloating diffusion region and the second floating diffusion region, andthe first transfer transistor does not operate.
 6. A complementarymetal-oxide-semiconductor (CMOS) image sensor comprising: a plurality ofpixels, wherein each of the pixels includes a first floating diffusionregion, in which image information is recognized over a wide range ofilluminance, and a second floating diffusion region, in which imageinformation is recognized with high sensitivity, and signals acquiredfrom the first floating diffusion region and the second floatingdiffusion region are synthesized, to thereby provide an image signal forthe pixel, and the first floating diffusion region is disposed between afirst transfer transistor and a first reset transistor and the secondfloating diffusion region is disposed between a second transfertransistor and a second reset transistor, and the first and secondfloating diffusion regions are not connected to a capacitor.
 7. The CMOSimage sensor of claim 6, wherein at least two pixels among the pluralityof pixels share the first floating diffusion region and the secondfloating diffusion region.
 8. The CMOS image sensor of claim 6, whereina first gate-driving voltage of a transistor, which generates acorresponding potential in the first floating diffusion region, isdifferent from a second gate-driving voltage of a transistor, whichgenerates a corresponding potential in the second floating diffusionregion.
 9. The CMOS image sensor of claim 6, wherein: the first floatingdiffusion region has a capacitance larger than the second floatingdiffusing region.
 10. The CMOS image sensor of claim 6, wherein: thesignal acquired from the first floating diffusion region is outputthrough a first drive transistor and a first select transistor, thesignal acquired from the second floating diffusion region is outputthrough a second drive transistor and a second select transistor, andthe acquired signals are then synthesized to provide an image signal.11. A complementary metal-oxide-semiconductor (CMOS) image sensorcomprising: a first light-receiving unit to convert light into charge; afirst floating diffusion region, in which a first potentialcorresponding to the converted amount of charge is generated; and asecond floating diffusion region, to which the charge in the firstfloating diffusion region is transmitted, and in which a secondpotential is generated, wherein a wide dynamic range signal is acquiredfrom the first floating diffusion region, a high-sensitivity signal isacquired from the second floating diffusion region, and the acquiredsignals are synthesized and output, and the first floating diffusionregion has a capacitance larger than the second floating diffusingregion.
 12. A complementary metal-oxide-semiconductor (CMOS) imagesensor comprising: a plurality of pixels, wherein each of the pixelsincludes a first floating diffusion region, in which image informationis recognized over a wide range of illuminance, and a second floatingdiffusion region, in which image information is recognized with highsensitivity, and signals acquired from the first floating diffusionregion and the second floating diffusion region are synthesized, tothereby provide an image signal for the pixel, and the first floatingdiffusion region has a capacitance larger than the second floatingdiffusing region.